This invention relates to the determination of shade.
Particles such as gemstones, particularly diamonds, can be sorted according to their colour and their shade. For example, between the colours white and black there is a variety of shades of grey, all of which have the same relative proportions of the primary colours. The difference between a white surface and a grey surface is that the former reflects most of the light which falls on its whereas the latter absorbs an appeciable fraction of the light, and reflects the rest.
In the diamond industry increasing use is being made of machinery for automatically sorting diamonds according to their colour. A colour determination of a diamond can be effected, for example, by determining the relative intensities of the primary colours reflected by the diamond. Such a determination is, however, independent of shade.
One way in which the shade of a particle is assessed is to compare the particle against differently shaded illuminated backgrounds. When a particle of a particular shade is viewed against a background of the same shade the particle apparently vanishes. If this viewing is done electronically, for example, by means of a photodetector which is responsive to the intensity of light on the background, it is found that there is no change in the detector output when the particle and the background are of the same shade. However, if the particle is not of the same shade as the background the output signal of the detector changes in amplitude. The change in amplitude of the detector signal is a function of the shade of the particle and of the size of the particle. In existing methods of shade sorting no attempt is made to record the actual amplitude of the signal produced. The sort is made simply on the basis of whether the output signal amplitude changes positively or negatively relative to a null value. Hence the fact that the amplitude of this signal is size dependent is not of any consequence.
The particles which cause a positive or a negative signal can be ejected at each pass, their shades being lighter or darker than the background in use. Hence one pass through the sorting machine and a different background is required to sort each shade.